1. Field of the Invention
This invention relates generally to a process for the production of sealed anodized aluminum and, more particularly, to a process that enables enhanced hydrothermal sealing of the anodic oxide coating formed on aluminum substrates.
2. Description of the Prior Art
Anodic oxide coatings are established upon aluminum or aluminum alloy substrates for various purposes including those of improving resistance to corrosion and abrasion. These coatings are formed by various conventional methods. For example, the anodic coating may be formed by anodizing (passing electric current through the treating solution with the substrate being coated serving as the anode) in an acid medium such as a sulfuric acid solution or a sulfuric acid containing sulfophthalic acid solution according to well known procedures.
In most current commercial practice direct-current anodizing in a sulfuric acid-based electrolyte has substantially replaced most other anodizing processes for the production of thick, clear, porous-type anodic oxide coatings, because of its efficiency in consumption of electrical current as compared with earlier alternating current processes. In general, direct current anodizing voltages employed for sulfuric acid-based electrolytes range from 12 to 22 volts depending upon the strength and temperature of the acid. Sulfuric acid-based electrolytes include mixtures of sulfuric acid with other acids, such as oxalic acid and sulphamic acid, in which the anodizing characteristics are broadly determined by the sulfuric acid content. Typically in sulfuric acid anodizing the electrolyte contains 15-20% (by weight) sulfuric acid at a temperature of 20.degree. C. and a voltage of 17-18 volts.
The coatings produced by the foregoing methods have no color. They are often referred to as clear anodized coatings. Such coatings can, however, be colored by various well known procedures including dying, hard color anodizing and electrolytic deposition.
Coloring by electrolytic deposition of inorganic particles has become particularly well known. In the electrolytic deposition process, inorganic material is deposited in the pores of the anodic oxide coating by the passage of electric current (usually alternating current) between the anodized aluminum substrate and a counterelectrode, while the anodized substrate is immersed in an acidic bath of an appropriate metal salt. The most commonly employed electrolytes are salts of nickel, cobalt, tin and copper. The counterelectrode is usually graphite or stainless steel, although nickel, tin and copper electrodes are also employed when the bath contains the salt of the corresponding metal.
The production of anodic oxide coatings, both clear and colored, can be performed with either batch or continuous operations. Batch operations are particularly adopted for anodizing small individual articles. In a batch operation the article to be coated is first anodized generally by immersing for a given period of time in an anodizing bath and then, if color is desired, the article is subsequently immersed in a coloring bath. Continuous operations are particularly adapted to anodizing strip or coiled aluminum substrates. In continuous operations the strip or coil is continually passed through the anodizing bath and, if desired, subsequently through a coloring bath. Known methods for batch anodizing are disclosed in U.S. Pat. Nos. 3,382,160; 3,616,297; 3,616,308; 3,616,309; and 3,622,471. Methods for practicing continuous anodizing are shown in U.S. Pat. Nos. 3,359,189; 3,359,190; 3,471,375; 3,535,222; and 3,718,547.
Anodic oxide coatings established on aluminum substrates are generally comprised of substantially anhydrous aluminum oxide. These coatings or films are relatively hard, porous and highly absorbent. For most purposes the substantially anhydrous coating as established on the metal substrates is found unsatisfactory. However, these characteristics can be markedly improved by a process hereinafter referred to as "sealing".
Sealing is basically a hydrothermal process wherein the formed, porous aluminum oxide coating combines with water at temperatures which enhance the formation of the hydrated oxide material. Sealing is believed to consist primarily of the conversion of substantially anhydrous aluminum oxide to various hydrated products with the attendant swelling or volume increase which is effective to partially close or "seal" the pores thereby diminishing the surface area of the coated surface. Sealing thus reduces the absorbency of the coated material rendering it more impervious. Poor sealing results in an inferior anodized product which tends to stain and "bleed" (if colored by certain processes).
In conventional sealing of anodized aluminium the alumina at the walls of the pores in the oxide film is partially hydrated by contact with hot water (usually 80.degree. C.--boiling point) held at a pH of 5.5-6.5. This hydration swells the alumina and causes the pores to become essentially filled with partially hydrated alumina. Regardless of the composition of the anodic coating, the solids formed by the hydrothermal treatment are aluminium hydroxide gel, pseudoboehmite, and crystalline boehmite.
An often objectionable side effect of the sealing process is a noticeable and undesirable change in the surface appearance of the anodized coating which has been found to be caused by the formation of a residual layer of loose crystalline boehmite on the surface of the anodic film that often appears iridescent or velvety. This so-called "smut" (sometimes referred to as "smudge") is an especially severe problem with colored anodic coatings.
Various post-sealing treatments have been proposed for removing smut including wiping, and spraying or dipping in mineral acid. None of these procedures have, however, been found to be acceptable. Wiping is time consuming and labor intensive, and, consequently, not commercially desirable. Mineral acid treatments are undesirable in that in many instances smut removal has been found to be incomplete. Additionally, in some instances the acid detrimentally affects the degree and quality of the seal.
Unfortunately once smut is formed during sealing, it cannot be removed except by mechanical or chemical means. Consequently, various proprietary anti-smut additives have been developed and marketed to suppress the initial formation of smut during the sealing process. These additives are incorporated into the sealing bath and generally function by suppressing the formation of crystalline boehmite particles on the surface of the anodic oxide coating while still allowing hydration to take place in the pores of the coating, particularly at the mouth of the pores. The sealing quality attained using baths containing such additives has been found to be satisfactory. Such additives are ineffective, however, in removing smut once it has been formed.
Examples of anti-smut additives for incorporation in sealing baths for anodic oxide films are described in British Pat. Nos. 1,265,424; 1,302,288; 1,368,336; 1,398,589 and 1,419,597. Examples of commercially available anti-smut additives are Henkel VR/6252/1, Henkel VR/6253/1 and Sandoz Sealing Salts A/S.
While minor traces of boehmite particles can upon close inspection usually be detected on the surface of coatings produced from such additive-containing baths, such coatings are generally considered to be "smut-free".
In many anodizing plants the sealing stage has been found to be a bottleneck in the process, because of the relatively long period of time involved to effect a seal of good quality. In conventional hot water sealing the time required to effect such a seal is generally about 2 to 3 minutes per micron of film thickness. Consequently, the time required to seal a load of anodized work having an anodic oxide coating of 25 microns in thickness may be an hour or more. Moreover, the cost, due to energy consumption, of maintaining hot water baths at or near their boiling points for periods of time longer than necessary continues to become increasingly prohibitive.
Sealing accelerators have therefore been developed which when added directly to the sealing bath accelerate the sealing process. Thus it is known that the sealing process may be accelerated by the addition of accelerators directly to the hot water sealing bath. Such accelerators are usually mildly basic substances which raise the alkalinity of the sealing bath to a value in the range of pH 7 to 11. U.S. Pat. Nos. 3,365,377 and 3,822,156 disclose the addition of triethanolamine to hot water sealing baths to accelerate sealing.
Generally, raising the pH of the sealing bath has been found to accelerate the formation of boehmite. The formation of boehmite is accelerated in the pore mouths but unfortunately is also accelerated on the surface of the film also. While the addition of TEA to a sealing bath free of anti-smut additive reduces the sealing time to about 1 min/micron film thickness, it also gives rise to a level of smut formation which is unacceptably high. Thus as for example in U.S. Pat. No. 3,822,156, sealing in the presence of an accelerator requires a post sealing smut removal step. Attempts to remove the smut formed during sealing in the presence of an accelerator have not been entirely satisfactory. While in some instances reasonable amounts of smut have been removed in this manner, sometimes smut removal has been incomplete, and, on other occasions, employing the smut removal step has been detrimental to the degree and quality of the seal.
Anti-smut additives cannot generally be used in a sealing bath containing an accelerator. Thus various methods have been tried to gain the expedience of the accelerated seal without attendant smut build-up. For example, temperatures have been lowered in the sealing bath containing the accelerator. While this method seems to reduce the smut formation, the seal quality has proven poor resulting in staining due to open pores in the anodic oxide coating.